Flowable metal powders



United States Patent 3,481,714 FLOWABLE METAL POWDERS John H. Harrington, Warwick, and Arnold L. Prill, Sufiern, N.Y., assignors to The International Nickel Company, Inc, New York, N.Y., a corporation of Delaware Original application Sept. 26, 1966, Ser. No. 581,768, now Patent No. 3,397,057, dated Aug. 13, 1968. Divided and this application Mar. 4, 1968, Ser. No. 729,849 lint. Cl. B22f 1/00 US. Cl. 29-182 5 Claims ABSTRACT OF THE DISCLOSURE Directed to flowable metal powders in the form of substantially spherical agglomerates having a particle size of about 20 to about 1,000 microns and having an internally sintered structure made of a plurality of fine metal powder particles.

The present application is a division of our copending US. application Ser. No. 581,768 filed Sept. 26, 1966, now Patent No. 3,397,057.

The present invention solves a problem which has existed in the art for seventy years.

Metal powders produced by the thermal decomposition of metal carbonyls, e.g., nickel and iron powders so produced, have been available since the days of Ludwig Mond. Such metal powders are valuable metallurgical raw materials because of their high purity. The powders have good pressability, provide compacts having high green strength and green compacts made thereof can be sintered to high density at relatively low temperatures as compared to coarser nickel powders produced by other methods.

However, the physical nature of these powders is such that they do not flow readily. This lack of flowability prevents utilization of carbonyl metal powders in conventional pressing equipment employed in the powder metallurgy art, despite the other marked advantages of these powders. The as-produced powders not only fail to fill dies in automatic pressing equipment, but, also, because of their fineness, quickly cause scoring and seizing of the punch within the die.

Carbonyl iron and nickel powders have average particle sizes of less than microns and, more usually, less than 5 microns. Carbonyl iron powders generally tend to be spherical in shape and their poor flowability thus is principally due to the fine particle size thereof. Carbonyl nickel powders tend to be irregular in shape, with spiky projections extending from more or less spherical bodies and, in some grades, with an overall fibrous appearance. These physical attributes further contribute to poor flowability to the extent that a pile of the powder can be parted with a knife and the parted face removed leaving a substantially vertical face.

It will be appreciated that the flowability of a particular powder is an empirical factor. Measurements of relative flowabilities of various powders can be made by timing the passage of a measured quantity of powder through a funnel having an orifice with a standard size at the bottom. However, as pointed out by W. D. Jones in his work Fundamental Principles of Powder Metallurgy, 1960, at page 973, so many factors influence the manner in which powders flow that present-day flowability testing procedures are not completely satisfactory. The important test of flowability, in practice, is the rate at which powder can be made to flow from a hopper to a die in which it is to be compressed.

Flowable fine metal powders are usually produced industrially by three general techniques: (1) the metal or alloy is produced as a relatively brittle ingot which is See then crushed and pulverized; (2) the metal or alloy in nonflowing fine powder form is sintered to a cake which is then crushed; (3) molten metal or alloy is atomized by spraying a liquid stream thereof into a fluid medium. All of these techniques are characterized by commercial objections, including loss of material as unusable fines which must be reprocessed at extra cost, introduction of undesired impurities, including oxides, other compounds, tramp elements, etc., and by the fact that the product does not always have satisfactory flowability. Accordingly, despite many attempts in the art, no commercial method is presently available for increasing the flowability of essentially nonfiowable pure metal powders while still retaining the high purity, good pressability and good sinterability thereof.

We have now discovered a process for greatly improving the flow properties of fine metal powders having poor flow characteristics while at the same time preserving the purity and other desirable metallurgical properties of the powders.

It is an object of the present invention to provide a method for improving the flow characteristics of fine metal powders having poor flow characteristics.

It is a further object of the invention to provide a practical means for improving the flow characteristics of fine metal powders.

It is another object of the invention to provide a method for ag lomerating fine metal powders to improve the flow characteristics thereof without detrimentally affecting the other important metallurgical characteristics thereof.

Other objects and advantages of the invention will become apparent from the following description.

Generally speaking, the present invention is directed to a process for improving the flowability of fine metal powders having high purity and poor flowability comprising balling the metal powder with a liquid such as Water, then drying and sintering a bed formed from the resulting agglomerates in a protective atomsphere to a temperature range in which substantial sintering occurs within the agglomerates but below that at which substantial sintering occurs between agglomerates to produce substantially spherical, flowable agglomerates.

The invention is particularly applicable to the treatment of fine metal powders of high purity having a particle size not exceeding about 10 microns and bulk densities of about 0.5 to about 3.5 grams per cubic centimeter (gm./cc.). Metal powders such as an carbonyl iron, carbonyl nickel and carbonyl cobalt powders are particularly amenable to treatment in accordance with the invention to provide free-flowing particles having a particle size of at least about 20 microns to about 1,000 microns, e.g., about 20 to about microns, while still retaining the high purity and other desirable metallurgical qualities in these materials. Electrolytic copper and iron powders and powders obtained by the reduction of halides, e.g., the chlorides of nickel, copper, molybdenum, tungsten, etc., are also amenable to treatment in accordance with the invention.

It is to be appreciated that the wet agglomerates, which may contain, for example, about 5% to about 30% water, by weight, have low strength and it is important that they should be transferred immediately, or at least after the lapse of only a short time during which loss of water is prevented, to the sintering and drying operations with only minimal handling. This can be readily accomplished, for example, by loading the wet agglomerates directly from the balling operation to a continuous belt communicating with the drying and sintering furnace. Any other means whereby the wet agglomerates are formed into a substantially fixed, quiescent or static bed during drying and sintering may be employed.

The balling operation may be conducted in a balling disc or drum, a vibrating table, vibrating screen, etc., equipped with a liquid fog or spray feed, or in a rotary twin-cone blender equipped with a rotating liquid spray bar located near the juncture of the cones or in any other convenient type of balling equipment known to those skilled in the art. Spray drying may also be employed for agglomerating. We prefer the aforementioned twincone liquid-solids blender having a rotating liquid spray bar adapted to introduce liquid in the form of a spray or fog under substantial velocity and pressure into the tumbling powder. Blenders of this general type are described, for example, in US. Patents Nos. 2,890,027 and 2,915,300 and patents mentioned therein. Balling is conducted by tumbling the powder under conditions such that components of rolling and of compression are imparted thereto while the balling liquid is introduced into the tumbling powder. It is advantageous from the control standpoint to introduce the balling liquid in the form of fine droplets as a spray or fog into the dry powder while the powder is in motion in the balling apparatus. Difliculties are encountered in attempting to moisten the metal powder before balling and control of agglomerate size in balling is uncertain, particularly from the standpoint of size unformity.

The green strength of the agglomerates can be increased to permit more handling and some motion of the agglomerates during drying and sintering by incorporating a soluble or dispersible organic heat-decomposable binder in the liquid employed for wetting the powder. Thus, binders such as methyl cellulose, starch, gums, polyacrylamides, dexterines, etc., can be incorporated. While water, e.g., demineralized water, is preferred from the standpoints of operating ease and of maintaining product purity, volatile organic solvents, including carbon tetrachloride, trichorethane, ethyl and methyl alcohol, etc., can be employed in the balling operation and solvent-soluble binders such as parafiin, stearic acid, waxes, ethylcellulose, etc., can be dissolved therein. In balling, density and strength of the agglomerates are increased by continuing the balling operation for some time after ball formation is initiated.

As noted previously, it is preferred to employ demineralized water itself as the liquid medium in the balling operation. The water can be removed from the agglomerates during the drying and sintering operations without any detrimental impurities such as carbon, etc., being retained in the final agglomerates. In this way, the other desirable metallurgical characteristics of the original powder, including good pressability, sinterability at low temperatures and purity are retained. It is important that the agglomerates not be disturbed during the interval after drying and before sintering since they then have low strength. It is found that even when minor amounts of material such as methyl cellulose are employed as binders for the purpose of strengthening water-wetted agglomerates a carbon residue may result therefrom in the subsequent heat hardening or sintering operation and such residues may be undesirable in certain instances. The presence of water during sintering, e.g., a wet hydrogen atmosphere at 1700 F., promotes the decarburization of the nickel, iron or other metal powder. Since carbonyl powders do contain carbon, sintering in wet hydrogen permits purification by decarburizing and deoxidizing the powders during the sintering under reducing conditions in the presence of water. Carbon contents as low as 0.005% and lower, e.g., 0.001%, can readily be obtained in this manner.

The sintering operation is conducted in a protective atmosphere which may be, for example, hydrogen, cracked ammonia, partially-combusted natural gas, argon, etc. The essential requirement of the atmosphere in the heat hardening or sintering operation is that it prevents oxidation of the metal powder agglomerates being sintered. In

the case of agglomerates made of nickel, iron or cobalt powder, the sintering operation is conducted at a temperature not exceeding about two-thirds of the melting point of the metal as measured in degrees Fahrenheit. In sintering nickel powder, a temperature of about 1000 F. to about 1730 F., e.g., about 1200 F. to about 1500 F., for a time between a few seconds up to several minutes, e.g., up to 15 minutes, depending upon temperature is satisfactory. It is surprisingly found that the initial agglomerates substantially retain their size and shape as a result of the sintering operation but that there is little adhesion between the agglomerates. Any caking or interagglomerate adhesion is readily removed by light mechanical treatment with only minimal loss of material in the form of fine dust. It is found that when sintering of nickel powder agglomerates is conducted at temperatures below about 1000" F., e.g., 800 F., an unduly high proportion of fine material is obtained whereas at a temperature of about 1500 F. only a small amount of fine material resulted. At temperatures exceeding two-thirds of the metal melting point in degrees Fahrenheit, interagglomerate bonding becomes predominant, control of agglomerate size is lost and the product becomes tough and ductile with loss of the desired pressability and relatively low temperature sinterability greatly desired for powder metallurgical operations. Again, with reference to carbonyl nickel powder agglomerates, a sintering temperature in the range of about 1200 F to 1300 F. provides sintered agglomerates which do not break down on handling yet have the desired pressability and sinterability for pressing and sintering, direct powder rolling and other powder metallurgical operations. A sintering temperature of about 1500 F. to 1700 F. provides tough, free-flowing agglomerates useful in other applications such as tubular welding electrodes, seed material in fluid bed carbonyl decomposers, etc. Similar results are obtained with other metal powders such as carbonyl iron and carbonyl cobalt powders by sintering at temperatures comprising equivalent proportions of the melting points in degrees Fahrenheit for these metals.

In a further advantageous aspect of the invention, it is found that pulverizing the sintered agglomerates, for example, in a hammer mill, provides a further improvement in flowability, i.e., reduction in flow time in a flowmeter, an increase in apparent density and an improved capacity to be directly rolled to strip having higher apparent density. As applied to fine carbonyl nickel powders, wateragglomerated balls sintered in hydrogen at temperatures of about 1400 F. to about 1730 F. are particularly suitable for pulverization to improve the flow properties thereof. Carbonyl nickel powder agglomerates sintered at temperatures above 1730 F. cannot readily be pulverized. Carbonyl iron powder agglomerates and codeposited 50% iron-50% nickel carbonyl powder agglomerates sintered at temperatures circa 1700 F. for 10 minutes in hydrogen displayed improved flow rate and increased apparent density after pulverization.

In order to give those skilled in the art a better appreciation of the advantages of the invention, the following illustrative examples are given:

EXAMPLE I About 2,000 grams of nickel powder produced by the decomposition of nickel carbonyl and having an average particle size in the range of about 3 to 5 microns with an apparent density in the range of about 1.6 to 2.1 gm./cc. were water agglomerated in a liquid-solids blender of the twin-cone type equipped with a high speed liquid spray feed bar. About 350 milliliters of water were employed in the operation and the powders were simultaneously tumbled and blended during the wetting to achieve agglomeration. Portions of the wetted agglomerates were placed in a metal boat and were dried and sintered at 1150 F., 1200 F. and 1250 F. in a furnace having a hydrogen atmosphere for about 5 minutes. In each case, free-flowing agglomerates having substantial strength were obtained with only a minimum amount of fines. The agglomerated powders were compacted and sintered to evaluate response in conventional powder metallurgy processing. As compared to the original powder, there was only a slight decrease in strength and ductility of the resulting sintered compacts but it was found there was an offsetting beneficial reduction in the amount of shrinkage which occurred during sintering. A portion of the agglomerates sintered at 1200 F. in hydrogen had a flow rate of about 50 seconds as determined in the Hall Flowmeter described in A.S.T.M. Standard B-213 whereas the initial powder would not pass through the flowmeter. The material had an apparent density of about 1.7 gm./ cc. and was directly cold roll-compacted to strip having a density about 60% of theoretical with no lamination in the roll-compacted strip being evident. Another portion of the agglomerates sintered in hydrogen at 1500 F. for 5 minutes had a flow rate of 52.75 seconds in the aforementioned Hall Flowmeter and an apparent density of about 1.7 gm./cc. This material was pulverized in a hammer mill Whereupon the flow rate was increased to 30 seconds and the apparent density was increased to 2.7 gm./cc. It was directly cold roll-compacted to strip having a density about 80% of theoretical with no lamination in the roll-compacted strip being evident. Another portion of the agglomerates sintered in hydrogen at 1700 F. for 5 minutes had a flow rate of 45 seconds and an apparent density of about 1.9 gm./cc. All of the sintered agglomerates had reduced surface area and gas content as compared to the original powder.

EXAMPLE II Two types of carbonyl iron powder having, respectively, an average particle size of about 5 microns and about 6 microns and an apparent density of about 3 gm./cc. and about 3 gm./cc. were agglomerated with water in the manner described in conjunction with Example I and were sintered in hydrogen at a temperature of about 1250 F. for about 5 minutes. The flow rate of the resulting material was about 46.9 seconds whereas the initial powder would not pass through the fiowmeter. The apparent density of the resulting material was about 2.12 gm./ cc.

EXAMPLE III The invention also contemplates agglomerated metal powders, especially carbonyl nickel, cobalt and iron powders and mixtures and alloys thereof, having a particle size of at least about 20 microns and up to about 1,000 microns, having good flowability, e.g., a flow rate of about 25 to about 50 seconds in the standard Hall Flowmeter described in A.S.T.M. Standard B213, and having an apparent density of about 1.5 to about 3 or 4 grams per cubic centimeter. With particular reference to fine carbonyl nickel powders, agglomerates having an apparent density in the range of about 1.7 to about 2.7 grams per cubic centimeter are readily provided. A further characteristic of the agglomerates is that they are compactible and may readily be hot or cold roll-compacted directly to strip without lamination of the strip. The agglomerates have an irregular particle outline.

In contrast to the results achieved in accordance with this invention, portions of the same carbonyl nickel powder described in conjunction with Example I were sintered in hydrogen to form cakes. A temperature in the range of 1700 F. to 2000 F. was required to produce sintered cakes from the loosely packed powder. The cakes were quite tough and ductile. When the cakes were mechanically crushed it was found that the resulting crushed powder aggregates were irregularly shaped and they exhibited poor flow characteristics. Again, when carbonyl nickel powder of the same type as that described in conjunction with Example I is sintered in hydrogen without agglomeration at a temperature of about 1200 F. and the resulting sintered material is crushed, the powder obtained has a size distribution similar to that of the initial powder and is not improved in flow rate. However, material processed with agglomeration in accordance with the invention and sintered at 1200 F. is a free-flowing agglomerated powder. The foregoing confirms that the invention affords a method for producing free-flowing metal powder starting with materials such as fine carbonyl nickel powder having a poor flow characteristic wherein only a low energy input is required. This feature provides economy in carrying out the process of the invention.

It will be appreciated that not only can powders of a single metal be agglomerated in accordance with the invention to provide free-flowing powder agglomerates but that also carbonyl codeposited iron-nickel powders, alloyed powders, coated powders and mixtures of initial single metal powers can be treated so as to produce powder agglomerates containing controlled proportions of the initial metals. Thus, agglomerates containing nickel and iron; nickel and cobalt; nickel, iron and cobalt; nickel and copper, etc., can readily be produced in accordance with the invention an these materials can be employed directly to produce alloy articles by pressing and sintering in accordance with conventional powder metallurgy techniques.

Furthermore, the process provided in accordance with the invention can be employed to provide coated powders. Thus, nickel coatings can be produced upon other powders such as chromium, graphite and copper by agglomerating and sintering in accordance with the invention. It will be appreciated that when such a procedure is employed, the particles to be coated may be of major particle sizes, e.g., about to about 100 microns, and these can be coated with fine particle size powders by wet agglomeration and sintering as described hereinbefore. The invention is also applicable to the production of freeflowing materials for use in dispersion-hardening systems. Thus, the agglomerating liquid may contain therein a salt which is heat-decomposable to a stable oxide such as thoria, alumina, etc., to provide thorough wetting of the initial powder with a salt such as, for example, thorium nitrate. During the sintering operation as described hereinbefore, the salt decomposes to provide finely dispersed material intimately mixed throughout the hardened agglomerates. Oxides, carbides, nitrides, silicides and other dispersants can be introduced into nickel an other metal powders in this manner by wet agglomeration with a solution of an appropriate decomposable salt.

We claim:

1. As a new article of manufacture, free-flowing, sintered metal powder agglomerates having a substantially spherical shape and composed of at least one metal from the group consisting of nickel, iron, cobalt and copper, said agglomerates having an average particle size of about to about 1,000 microns and being made of metal particles less than ten microns in average particle size sintered together, said agglomerates being characterized by an apparent density of about 1.5 to about 4 grams per cubic centimeter and a flow rate as measured by the Hall Flowmeter of about to about seconds.

2. A carbonyl iron powder agglomerate according to claim 1.

3. A carbonyl cobalt powder agglomerate according to claim 1.

7 8 4. A carbonyl nickel powder agglomerate according to FOREIGN PATENTS claim 1. 34 3 1 5. A carbonyl nickel powder agglomerate according to $33 2 4 32; 82:2: claim 4 having an apparent density of about 1.7 to about 818191 8/1959 Great Britain 2.7 grams per cubic centimeter, and an average particle size of about 20 to about 150 microns.

References Cited CARL D. QUARFORTH, Primary Examiner UNITED STATES PATENTS ARTHUR I. STEINER, Assistant Examiner 2,776,200 1/1957 Wallis 75-5 10 CL XR' 2,844,456 7/1958 Llewelyn 75.5 2,851,348 9/1958 Oestrcicher 75-.5 75.5, 211 2,857,270 10/1958 Brondin 75213 2,935,394 5/1960 Hiler 75.5 

